Metal-halide discharge lamps

ABSTRACT

A metal-iodide discharge lamp with two main and one auxiliary electrodes mounted in a quartz discharge tube. Deterioration of the quartz between the metal-to-quartz seals of the auxiliary electrode and the adjacent main electrode is substantially prevented by a thermal switch which keeps the auxiliary electrode disconnected from the remote main electrode while the discharge tube is running at its normal operating temperature.

United States Patent 1 Page et al.

{451 Feb.6,1973

I54] METAL-HALIDE DISCHARGE LAMPS [75] Inventors: Robert Brian Page; Thomas William Cartwright, both of London, En-

gland [73] Assignee: Thorn Electrical Industries Limited, London, England [22] Filed: May 11, 1971 [21] App1.N0.: 142,313

[30] Foreign Application Priority Data June 26, 1970 Great Britain ..3l,200/70 [52] US. Cl. ..315/60, 315/127, 313/220, 313/229, 313/198 [51] Int. Cl ..H01j7/30, HOlj 17/30 [58-] Field of Search....3l5/60, 72, 47, 124, 126, 225, 315/226, 313/229, 225, 220, 197-198 56] References Cited UNITED STATES PATENTS 3,619,710 11/1971 Waymouth ..315/47 3,234,421 2/1966 Reiling ..313/25 3,514,659 5/1970 Gungle et a1. ..313/229 X 3,398,312 8/1968 Edris et a1. ..313/225 3,407,327 10/1968 Koury et a1. 13/225 X 3,226,597 12/1965 Green ..315/60 3,307,069 2/1967 Fraser et al ..315/60 3,445,719 5/1969 Thouret et a1 ..313/229 X FOREIGN PATENTS OR APPLICATIONS 1,008,339 12/1965 Great Britain ..313/228 Primary Examiner-Herman Karl Saalbach Assistant ExaminerMarvin Nussbaum Att0rney.lames Theodosopoulos [57] ABSTRACT A metal-iodide discharge lamp with two main and one auxiliary electrodes mounted in a quartz discharge tube. Deterioration of the quartz between the metalto-quartz seals of the auxiliary electrode and the adjacent main electrode is substantially prevented by a thermal switch which keeps the auxiliaryelectrode disconnected from the remote main electrode while the discharge tube is running at its normal operating temperature.

15 Claims, 1 Drawing Figure PATENTEDFEB 6 I973 ROBERT BRIAN PAGE THOMAS WILLIAM CARTWRIGH'T INVENTORS BY W ATTORNEY METAL-HALIDE DISCHARGE LAMPS BACKGROUND OF THE INVENTION This invention relates to metal halide discharge lamps.

Metal-halide discharge lamps have been developed in an attempt to provide discharge lamps in which the portion of space within the discharge tube which is not effectively used in producing light is reduced. A description of some examples of such lamps is given at pages 283 to 286 of Lamps and Lighting, edited by H. Hewitt and AS. Vause, and published by Edward Arnold (Publishers) Ltd., London, in 1966. Such lamps have-two main electrodes and one auxiliary electrode mounted in a quartz discharge tube, one of the main electrodes and the auxiliary electrode being adjacent one another and remote from the other main electrode. Usually each electrode is connected to a respective molybdenum foil metal-to-quartz seal embedded in the quartz of the discharge tube and serving to connect the respective electrode to circuitry outside the discharge tube.'Hitherto it has been arranged that the auxiliary electrode remained connected, albeit through a current-limiting impedor, to the power-input conductor which is connected to the main electrode which is remote from the auxiliary electrode. It has been found that with this arrangement, especially for lamps dissipating 1000 watts or more, the quartz between the molybdenum foils of the auxiliary electrode and the adjacent electrode deteriorates at an unacceptably high rate. The present invention has as an object the provision of a metal-halide discharge lamp in which such rapiddeterioration does not occur.

Another object of the invention is to provide a metalhalide discharge lamp with a commercially acceptable life and capable of dissipating 1000 watts under normal operating conditions.

1 SUMMARY OF THE INVENTION remote from the auxiliary electrode, and the other input conductor being connected directly to the other main electrode which is adjacent the auxiliary electrode which is connected to the said one conductor through a thermal switch and a current-limiting impedor. The thermal switch, which is a switch which opens at a first temperature thereof and closes at a second temperature thereof which may be the'same as the first temperature, may be a bi-metallic switch.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a schematic represen-' tation of a discharge lamp according to the invention and a ballast reactance to which the lamp is connected.

DESCRIPTION OF THE PREFERRED EMBODIMENT in the accompanying drawing there is shown schematically a 1000 watt MBI lamp l0 and its currentlimiting ballast reactance 1 1 and power-factor-correcting capacitor 12. MBI signifies that the lamp is a mercury linear quartz arc tube (high pressure) integral lamp. The lamp 10 embodies the present invention, the auxiliary electrode 13 of the lamp being connected to one end of a current-limiting resistor 14 through a bimetallic switch 15. The resistor 14 may present from 9 to 30 Kilohms resistance.

The lamp 10 has an outer envelope of borosilicate glass 16 and the space between this envelope and the quartz arc tube 17 is filled with nitrogen. Metal-toquartz seals are provided in the connections to the main electrodes 18 and 19 and the auxiliary electrode 13 of the tube in the form of molybdenum foils 20. Two power input leads 21 and 22 pass into the outer envelope 16 from a connector cap (not shown) of the Goliath Edison screw type directly to the foil 20 of the main electrodes respectively 18 and 19, one lead 22 also being connected directly to the other end of the current-limiting resistor 14.

The ballast reactance 11 and capacitor 12 to which the lamp 10 is shown connected are respectively an AME 53247.4 high reactance and a micro-farads capacitor, the lamp 10 being intended to be operated from a 240 volt a.c. mains supply.

The filling in the arc tube 17 has the following constituents in the ranges given:

Mercury 50 to milligrams Mercury iodide about 10 milligrams Sodium iodide 10 to 50 milligrams Scandium metal 0.1 to 10 milligrams Argon 10 to 55 millimeters of mercury pressure.

In operation, the estimated pressure within the tube 17 is approximately 3 atmospheres, the voltage drop across the tube 17 averages 250 volts, and the current through the tube 17 is approximately 4.2 amps. The ballast reactance 11 will transform 240 volts into 574 volts when the lamp 10 is presenting an open circuit thereto, and will limit the current to 5.8 amps when the lamp 10 is presenting a short circuit thereto.

ln starting the lamp 10 the bi-metallic switch 15, which is initially in its closed state, permits the auxiliary electrode 13 to function in the usual manner. The temperature of the lamp 10 begins to rise when the auxiliary electrode 13 is functioning and eventually a predetermined temperature is reached at which the switch 14 opens, discharge between the main electrodes l8 and 19 having by then been established.

When the lamp 10 is switched off, it cools and the bimain electrode adjacent thereto by the closure of a thermal switch when the lamp reaches its operating temperature, the molybdenum foils which seal the auxiliary electrode and the adjacent main electrode into the quartz arc tube are attacked chemically. Thischemical attack is thought to be electrolytic.

In embodiments of the present invention the arrangement of the thermal switch in relation to the electrodes is such that after the lamp has started, the auxiliary electrode is disconnected from the main electrode remote thereof andit is found that the chemical attack mentioned hereinbefore where molybdenum sealing elements are used is substantially eliminated.

In embodiments of the present invention where the discharge lamp has its discharge tube mounted in an outer envelope and the thermal switch is mounted within the outer envelope, as in the embodiment shown in the accompanying drawing, it is preferred that the thermal switch be positioned sufficiently close to the discharge tube so that the switch will assume the temperature of the discharge tube whatever-the orientation of the lamp duration operation.

We claim:

1. A metal-iodide discharge lamp comprising:

a quartz discharge tube;

two main electrodes;

an auxiliary electrode;

two power-input conductors;

a thermal switch;

and a current-limiting impedor;

the said electrodes being mounted in the discharge tube with'one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode;

one of the said power-input conductors being connected to the said one main electrode;

the other power-input conductor being connected to said other main electrode, and through the thermal switch and the current-limiting impedor to the auxiliary electrode, the said impedor being in series with the said switch; and said auxiliary electrode being electrically isolated fromsaid main electrodes upon opening of said thermal switch.

2. A lamp as claimed in claim 1, wherein the thermal switch is a bi-metallic switch.

3. A lamp as claimed in claim 1, wherein the currentlimiting impedor is a resistor.

4; A lamp .as claimed inclaim 1, wherein the discharge tube contains proportions of mercury iodide and sodium iodide.

5. A discharge lamp comprising:

a quartz discharge tube;

two main electrodes;

an auxiliary electrode;

two power-input conductors;

a thermal switch;

and a current-limiting impedor; the said discharge tube containing a porportion of at least one metal halide; the said electrodes being mounted in the discharge tube with one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode; 7

one of the said power-input conductors being connected'to the said one main electrode; the'other power-input conductor being connected to the said other main electrode, and through the thermal switch and the current-limiting impedor to the auxiliary electrode, the said impedor being in series with the said switch; and

said auxiliary electrode being electrically isolated from said main electrodes upon opening of said thermal switch.

6. A discharge lamp as claimed in claim 5, wherein the or each metal halide is a metal iodide.

7. A discharge lamp as claimed in claim 6, wherein the or at least one metal iodide is an alkali metal iodide.

8. A discharge lamp as claimed in claim 5, wherein the lamp consumes 1000 watts when operating.

9. A metal iodide discharge lamp comprising:

an outer envelope of borosilicate glass;

a quartz discharge tube mounted inthe outer envelope;

an insert gas filling in the space between the said outer envelope and the discharge tube;

two main and one auxiliary electrodes mounted in the discharge tube with one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode;

two power-input conductors passing into the said outer envelope with one being connected to the said one main electrode, and the other being connected to the said other electrode;

a current-limiting impedor;

a thermal switch mounted adjacent the discharge and connected tothe said impedor with the said other power-input conductor being connected to the auxiliary electrode through the said'impedor and the said switch in series;

said auxiliary electrode being electrically isolated from said main'electrodes upon opening of said thermal switch;

and the discharge tube containing a filling having as its constituents mercury, mercury iodide, sodium iodide, scandium metal, and argon.

10. A discharge lamp as claimed in claim 9, wherein the thermal switch is a bi-metallic switch which opens at a first temperature thereof and closes at a second of molybdenum, and the said auxiliary'electrode is connected to the said series connected switch and impedor through another metal-to-quartz seal formed of molybdenum.

13. A discharge lamp as claimed in claim 12, wherein the said metal-to-quartz seals are molybdenum foils.

14. A discharge tube as claimed-in claim 1, wherein the said one main electrode and the said auxiliary electrode are connected to respective metal-to-quartz seals formed of molybdenum.

15. A discharge tube as claimed in claim 5, wherein the said one main electrode and the said auxiliary electrode are connected to respective metal-to-quartz seals formed of molybdenum. 

1. A metal-iodide discharge lamp comprising: a quartz discharge tube; two main electrodes; an auxiliary electrode; two power-input conductors; a thermal switch; and a current-limiting impedor; the said electrodes being mounted in the discharge tube with one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode; one of the said power-input conductors being connected to the said one main electrode; the other power-input conductor being connected to said other main electrode, and through the thermal switch and the current-limiting impedor to the auxiliary electrode, the said impedor being in series with the said switch; and said auxiliary electrode being electrically isolated from said main electrodes upon opening of said thermal switch.
 2. A lamp as claimed in claim 1, wherein the thermal switch is a bi-metallic switch.
 3. A lamp as claimed in claim 1, wherein the current-limiting impedor is a resistor.
 4. A lamp as claimed in claim 1, wherein the discharge tube contains proportions of mercury iodide and sodium iodide.
 5. A discharge lamp comprising: a quartz discharge tube; two main electrodes; an auxiliary electrode; two power-input conductors; a thermal switch; and a current-limiting impedor; the said discharge tube containing a porportion of at least one metal halide; the said electrodes being mounted in the discharge tube with one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode; one of the said power-input conductors being connected to the said one main electrode; the other power-input conductor being connected to the said other main electrode, and through the thermal switch and the current-limiting impedor to the auxiliary electrode, the said impedor being in series with the said switch; and said auxiliary electrode being electrically isolated from said main electrodes upon opening of said thermal switch.
 6. A discharge lamp as claimed in claim 5, wherein the or each metal halide is a metal iodide.
 7. A discharge lamp as claimed in claim 6, wherein the or at least one metal iodide is an alkali metal iodide.
 8. A discharge lamp as claimed in claim 5, wherein the lamp consumes 1000 watts when operating.
 9. A metal iodide discharge lamp comprising: an outer envelope of borosilicate glass; a quartz discharge tube mounted in the outer envelope; an insert gas filling in the space between the said outer envelope and the discharge tube; two main and one auxiliary electrodes mounted in the discharge tube with one of the main electrodes and the auxiliary electrode adjacent one another and remote from the other main electrode; two power-input conductors passing into the said outer envelope with one being connected to the said one main electrode, and the other being connected to the said other electrode; a current-limiting impedor; a thermal switch mounted adjacent the discharge and connected to the said impedor with the said other power-input conductor being connected to the auxiliary electrode through the said impedor and the said switch in series; said auxiliary electrode being electrically isolated from said main electrodes upon opening of said thermal switch; and the discharge tube containing a filling having as its constituents mercury, mercury iodide, sodium iodide, scandium metal, and argon.
 10. A discharge lamp as claimed in claim 9, wherein the thermal switch is a bi-metallic switch which opens at a first temperature thereof and closes at a second temperature thereof.
 11. A discharge lamp as claimed in claim 9, wherein the said temperatures are substantially the same.
 12. A discharge lamp as claimed in claim 9, wherein the said one main electrode is connected to the said one conductor through a metal-to-quartz seal formed of molybdenum, and the said auxiliary electrode is connected to the said series connected switch and impedor through another metal-to-quartz seal formed of molybdenum.
 13. A discharge lamp as claimed in claim 12, wherein the said metal-to-quartz seals are molybdenum foils.
 14. A discharge tube as claimed in claim 1, wherein the said one main electrode and the said auxiliary electrode are connected to respective metal-to-quartz seals formed of molybdenum. 